Catalysts can be broadly characterised in three broad types, heterogeneous catalysts, homogeneous catalysts and hybrid homogeneous-heterogeneous catalysts. Heterogeneous catalysts act by catalysing a reaction at the boundary of two phases (e.g. a solid-liquid boundary). Homogeneous catalysts act by catalysing a reaction within the same phase as the reactants (e.g. both reagents and catalyst are dissolved in a liquid phase). Hybrid Homogeneous-Heterogeneous catalysts involve a homogeneous catalyst anchored to a supporting material. The resulting catalyst can be considered heterogeneous (from an engineering point of view) or homogeneous (from a chemical point of view).
Classical heterogeneous catalysts can be considered to include (i) bulk metal (e.g. a platinum mesh), (ii) metal oxides, sulphides, or carbides (e.g. iron oxide pellets), (iii) solid acids or bases (e.g. zeolites), (iv) supported metals (e.g. platinum on a silica support) and (v) alloys or discrete two-metal combinations (e.g. microscopic islands of ruthenium on a platinum base).
These catalysts are heterogeneous as catalysis occurs at a boundary between two phases such as the metal surface of the catalyst and a liquid or gaseous phase containing the reactant. The dominant interaction between catalyst and reagent during the catalytic process is adsorption. The catalyst consists of bulk (large) agglomerations of atoms connected to each other by metallic bonds or ionic bonds. The catalyst is a metallic or an ionic solid. Generally the catalyst is “multi-site”, that is it has many adsorption sites having different properties on the catalyst surface. Typically, the same type of atoms constitutes the adsorption sites, but the surface morphology of the catalyst affects the catalytic properties and outcome.
Most modern industrial catalytic processes involve this type of catalyst, usually in the form of metal or metal oxide plates, gauzes, or pellets which are placed in contact with reagent streams. In such systems, molecules of the reagents are constantly adsorbing onto the catalyst surface and then desorbing from it. If reagent molecules are adventitiously proximate and favourably disposed upon the surface following an adsorption process, their reaction with each other is facilitated and catalysed. If they are not, it is not. High temperatures and pressures, and the use of optimal catalyst materials and material structures are typically employed to increase the statistical likelihood of a successful encounter between reagent molecules on the catalyst surface. An important drawback of such catalysts is that they are usually unselective in their adsorption, meaning that chemical impurities in the feedstock streams also adsorb onto the catalyst surface. If these impurities do not desorb—as many do not—that site on the catalyst surface is poisoned. The multi-site nature of these catalysts can result in a mixture of products as a result of the different catalytic properties of the sites. However, despite these deficiencies such catalysts can and do provide an economic means for the manufacture desired products or for processing of reagents in an industrial setting. They are typically easy to separate from the end product by virtue of being in a solid phase and are often suitable for use in continuous manufacturing processes.
In contrast homogeneous catalysts involve reactions within a single phase by a molecular entity containing a catalytic group. Homogeneous catalysts can be considered to include classical homogeneous catalysts, biological catalysts and biomimetic catalysts.
Classical homogeneous catalyst includes (i) soluble acids and bases (e.g. HF), (ii) metal salts or metal complexes (e.g. Wilkinsons catalyst, Heck catalysts, metallocene catalysts), (iii) Radical initiators (e.g. benzoyl peroxide), (iv) certain solvents (e.g. dimethylformamide).
In all such systems there is no support or supporting substrate with the catalysis occurring in a single phase (typically within solution). The catalytic groups generally act in isolation from each other at the molecular level (i.e. normally “single-site”), i.e. they involve one catalytic group only. Typically the reagents bind to a single atom of the catalytic group (e.g. the metal atom in metallocene catalysts). These single site catalysts may also be referred to as mononuclear catalysts. In the rarer cases where the catalytic groups do not act in isolation (i.e. when two or more different catalytic groups are necessary), their proximity to each other during the catalytic process is not crucial. The intermediates formed with the catalytic groups are typically fairly stable.
Biological catalysts are enzymes per se and the catalysis occurs in a single phase in solution making it a homogeneous process. The enzymes do not use a support or supporting substrate. Biological catalysts exhibit a power and specificity which far exceeds the capacity of modern industrial catalysts. For example, one molecule of the enzyme carbonic anhydrase, which is one of the most active catalyst known, converts, on average, 100,000 carbon dioxide molecules each second (in our muscles) into aqueous carbonic acid (in our blood stream). It does this without fail, at body temperature and at a pressure less than 1 atmosphere. Moreover, it does this selectively in the presence of a wide variety of other possible reagents without becoming deactivated. By comparison, a typical modern industrial catalyst, such as the heterogeneous catalytic system of solid iron and oxide mix used in the important Haber-Bosch process for the production of ammonia from nitrogen and hydrogen, typically requires temperatures of 500° C. with the reagent gases compressed to 1,000 atmospheres of pressure. Even so, only 15-25% of the reagents typically convert to ammonia. The catalyst must also be periodically replaced because it is slowly rendered inactive by the presence of impurities in the reagent streams.
The origin of enzymatic activity and specificity has been the subject of numerous scientific studies. It is now generally agreed by practitioners of the art that enzymes perform as efficiently as they do because of two important properties:
(a) Proximate Binding: The active sites of many enzymes contain an arrangement of atoms which bind reagent molecules and hold them in close proximity and in favourable dispositions to each other. It does so by containing catalytic groups covalently bound to supporting structure having a particular configuration determined by intramolecular interactions between various atoms or groups within the structure. This proximity greatly facilitates reactions between the reagents by stabilising the required transition states. In so doing, the rate of the reaction may be increased by many orders of magnitude compared to the rate of the same reaction in the absence of the enzyme.
(b) “Lock-and-Key” Binding: The active sites of many enzymes only bind reagents whose shape and chemical properties are complementary. This “lock-and-key” interaction prevents the attachment of unwanted reagents which could temporarily or permanently block the active site. A reagent which permanently blocks the active site of a catalyst is said to “poison” the catalyst.
A key challenge in chemistry has been the design and preparation of artificial catalytic systems which mimic the action of enzymes either in principle or in practice. Such catalysts are known as biomimetic catalysts. They are homogenous catalysts in that they operate in a single phase. Such systems are also known as Artificial Enzymes, a term defined in the scientific article “Biomimetic Chemistry and Artificial Enzymes—Catalysis by Design” by R. Breslow in Accounts of Chemical Research (1995), Volume 28, pages 146-153. Two approaches have been largely employed to develop biomimetic catalysts by practitioners of the art:                (i) studying and elucidating the atomic structure of an enzyme's active site and then replicating it in an artificially constructed molecule.        (ii) constructing molecules containing, in close proximity to each other, atoms or units of atoms which are known to bind and stabilise reagents in a state conducive to reaction. These catalytic groups are different to those in the corresponding enzyme, but are known to be capable of performing similar functions.        
The molecules developed using approaches (i) and (ii) above rely on the proximity and disposition of catalytic groups to each other to bring about the catalysis. As is the case with enzymes, the catalytic groups are generally not individually catalytically active, but are transformed into highly active catalysts when in the correct proximity and disposition to other catalytic groups. This proximity is created in biomimetic catalysts of this type by covalently or coordinately binding the catalytic groups to the same molecular backbone or framework.
While certain molecules constructed using these strategies have been shown to produce catalytic properties similar to those of a corresponding enzyme, both of the above approaches have significant disadvantages:                (A) they require the chemical synthesis of often highly complicated molecules in which the catalytic groups (atoms or units of atoms) must be held in precise dispositions and proximities by covalent bonds between the catalytic groups. The exactness required in these parameters typically necessitates complicated synthetic chemical procedures which are labour intensive and expensive to perform.        (B) the artificial catalysts prepared in these approaches are discrete molecular units which must operate in open solution in the same way that enzymes do. This is not preferable from a manufacturing point of view because:        1. In many cases, the molecular scale of the catalysts makes them difficult to separate from the products they generate. This renders them unsuitable for continuous chemical manufacturing processes.        2. Catalysts which need to be continuously regenerated during their operation must be separately cycled to maintain their catalytic properties during continuous chemical manufacturing processes. This usually requires operating a separate set of “sacrificial” reagents in parallel to the manufacturing process.        
An example of the above difficulties is the catalytic conversion of protons (H+) such as present in acid, to hydrogen (H2). Hydrogen gas is an important energy-efficient and environmentally-friendly fuel which can, however, not be efficiently stored in the form of a gas. Its ready production, on demand, from acids by an economically feasible process therefore offers considerable opportunities. The reverse process, which involves the electrochemical conversion of hydrogen gas to acids, is additionally a convenient source of electrical energy which can be used to power electrical appliances. Naturally-occurring hydrogenase enzymes are efficient catalysts for converting acids (H+) to hydrogen gas, and the reverse process.
Several highly efficient artificial catalysts which mimic in principle or practice the mechanism of certain of the hydrogenase enzymes for the conversion of H+ to H2 have been discovered in recent years, with the compounds known as [1.1]ferrocenophanes emerging as amongst the most successful. In these complexes two ferrocene units are held in close and controlled proximity to each other by the presence of bridging methyl groups. This proximity (Fe . . . Fe distance=3.4-4.8 angstroms during twisting) facilitates a stabilizing interaction between two iron-bound H+ species which are present on each ferrocene when they are protonated. In acid solution, these H+ ions are therefore spontaneously converted to H2 and released by the ferrocenophanes which are themselves converted to di-ferrocenium ions in the process. The addition of a sacrificial reductant, such as lead, converts the di-ferrocenium ions back to [1.1]ferrocenophanes, thereby closing the catalytic cycle and allowing a continuous production of H2.
The problems associated with the commercial operation of these processes include the need to prepare the [1.1]ferrocenophanes by complicated synthetic procedures, the need to use strongly acidic solutions, and the difficulty of employing them in processes on a commercial scale. There is also a need for a sacrificial reductant.
Hybrid homogeneous-heterogeneous catalysts can provide a mixture of the advantages and disadvantages of homogeneous and heterogeneous catalysts, and thereby a partial solution to some of these problems. In particular, they offer the prospect of combining the advantages of homogeneous catalysts (greater selectivity and reproducibility in the catalysis, mild reaction conditions, ready chemical modification, and greater inherent efficiency) with the advantages of heterogeneous catalysts (ready incorporation in new and existing industrial processes, general ease of use) in a design which is inherently biomimetic.
The field of hybrid homogeneous-heterogeneous catalysts can be considered to include (i) immobilised classical homogeneous catalysts (e.g. Wilkinsons catalyst bound a polystyrene support), (ii) immobilised enzymes (e.g. an enzyme bound to a supporting substrate), (iii) biomimetic catalysts immobilised on the surface of a support or supporting substrate (e.g. [1.1]ferrocenophane bound to the surface of a support), and (iv) polymer-bound species which become catalytic groups only when in close proximity to each other.
These catalysts are typically associated with a supporting substrate by adsorption or by trapping of the catalyst on the substrate or by covalently binding or ion-paring one or more of the catalytic groups to a polymer backbone, where the polymer is anchored to a substrate. The catalytic activity of such hybrid heterogeneous-homogeneous catalysts are governed by a wide range of factors including the ability of the support to; (I) inhibit deactivation processes, (II) promote coordinative unsaturation of the metal atoms/ions in the catalytic groups, thereby speeding up the reaction, (III) improve the selectivity of the catalysis, with an accompanying improvement in the product properties, (IV) retain the catalytic groups strongly, allowing maximum product generation by the catalyst, and, as a consequence of the above factors, (V) allow cooperative multi-step catalysis in which the product of one step is consumed in the next.
One of the problems with using adsorption or trapping the catalyst to a support substrate is the generally poor durability of the catalyst. The use of ion pairing provides better durability but the polymer backbone is typically expensive to manufacture. The use of covalent bonding to anchor the catalytic groups to the polymer backbone significantly increases the manufacturing cost of this type of catalyst and it is believed that no such catalysts are being used in large scale in industry. This is unfortunate since locating suitable catalytic groups in close proximity to each other on a supported hybrid heterogeneous-homogeneous catalyst provides a prospectively useful means of mimicking biological catalysts in either principle or practice.
Thus there is a need for catalysts which mimic the action of biological catalysts whilst avoiding the manufacturing costs associated with such catalysts. Ideally, the catalysts would be capable of being used in continuous manufacturing processes and would therefore be of a hybrid heterogeneous-homogeneous type. To minimise the costs of such hybrid heterogeneous-homogeneous catalysts, it is desirable that a cheap support be used, along with a cheap method of incorporating the catalytic groups into or onto the support.